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Roadbeds supporting coastal highways in North Carolina are susceptible to erosion during large storm events. During large storms, such as hurricanes and nor’easters, storm surge and waves are able to erode the soil and undermine the highway. Coastal highways in North Carolina have experienced over-washing due to coastal storm surges, which led to pavement damage and even highway closure. Direct storm wave action on the seaward side of the highway and weir-flow damage on the landward side of the highway can undermine the roadbed, erode the supporting soil, and lead to pavement failure and road closure. In addition, slopes supporting roadways in sandy material are designed with a 3:1 (horizontal:vertical) slope due to the erodibility and stability of the material. More competent material may be designed with a 2:1 slope, thereby reducing the right-of-way extent. By reinforcing vulnerable coastal subgrades and slopes, erosion potential can be reduced and vital infrastructure can be maintained.
Bio-mediated soil improvement methods can be used to stiffen sandy subgrade and slopes and reduce the soil’s susceptibility to erosion. Natural biological processes have been shown to improve the behavior of sand deposits by increasing the sand’s strength, stiffness, and erosion resistance (Montoya and DeJong 2015, Lin et al. 2015, Shanahan and Montoya 2016). Bio-mediated soil improvement methods utilize natural soil bacteria to hydrolyze urea as a nutrient source and produce calcite cementation within the soil matrix. This bio-cementation process can be used to mitigate damage to highway subgrades and slopes by implementing the process in situ. The bio-cemented reinforced sand deposits would ultimately improve the resiliency of the coastal infrastructure by reducing the susceptibility to erosion in an innocuous, natural, and cost-effective manner.
In order to develop a bio-mediated method to reduce erosion susceptibility, a research program consisting of both laboratory and field work is proposed. The treatment process will be developed to improve the sandy subgrades and slopes and to be implemented by the Department. A relationship between the level of treatment and the resistance to erosion, shear strength, permanence and cost will be developed. Field test sections are proposed to assess the implementation and performance of the treatment method. The test sections, which will be treated and monitored, will take place within the right-of-way of one of the Department’s sites. The results of the field test sections will provide valuable information for developing a treatment method to be implemented by the Department.